Production of thermoplastic sheets of nonuniform thickness



Sept. 25, 1945. -r05 ET AL 2,385,486-

PRODUCTION OF THERMOPLASTIC SHEETS OF NONUNIFORM THICKNESS Filed March 28, 1942 Arm/wax Patented Sept. 25, 1945 STATES PATENT OFFICE PRODUCTION OF THERMOPLASTIC SHEETS OF NONUNIFORM THICKNESS Willard iF. Bartoe, Hulmeville, and Walter R. Speck, Langhorne, Pa.., assignors to Him & Haas Company, Philadelphia, Pa., a corporation of Delaware Application March 28, 1942, Serial No. 436,636

4 Claims.

This invention relates to the production of thermoplastic sheets of non-uniform thickness. It particularly relates to the casting of relatively large thermoplastic sheets or panels which are ine, notably by compression and injection molding. Larger objects such as those measuring several square feet in area cannot be produced conveniently by such technique. It is customary to produce such items by shaping sheets of thermoplastic materials. In such operation the sheets, which have been precast or pre-formed are heated until they become ductile whereupon they are shaped usually by application of pressure. Two convenient methods for carrying out this shaping operation are described in United States Patents Nos. 2,123,552 and 2,142,445, issued July 28, 1938, and January 3, 1939, respectively. Another method, which uses gaseous pressure instead of liquid pressure as in the former cases, is described in co-pending application, Serial No. 436,632, now United States Patent No. 2,367,642, filled March 28, 1942. In all such processes, the sheet is drawn to the desired shape and then cooled while being held in that particular shape. In the production of curved shapes or domes, some sections of the sheet are stretched or drawn more than others and as a result the walls of the domes are of uneven or non-uniform thickness.

lack of uniform thickness results in a loss oi strength at the thinner portions, and in the time of transparent domes it also results in a variation in the optical properties of the dome.

For example, in the production of such objects for aircraft, as airplane turrets, commanders domes, cockpit covers, bomber noses, transparent enclosures or windows for bombardiers stations, etc., it is highly desirable that the thermoplastic structural material be uniformly strong and free of optical distortion. This may be accomplished by having the walls of the domes, etc, of uniform thickness. Unfortunately, however, the drawing of uniformly thick, flat sheets to form these curved shapes results in non-unliorm sections as described above. For instance, if a flat circular sheet of uniform thickness is drawn into a transparent bomber-nose having a shape like the ordinary derby hat, the center of the original sheet is stretched much more than the portions nearer the perimeter. As a result, the sides of the dome are thicker than the top of the dome and, furthermore, the sides gradually taper in thickness from'the rim to the top. Consequently the top is much weaker than the sides and, depending on conditions, may be too weak to permit satisfactory use on aircraft.

Our invention provides a means of overcoming this difficulty by the production of thermoplastic sheets which are thicker in the center thanat the edges. For the sake of convenience we choose to refer to such non-uniform sheets as 'loaded or thick-centered" sheets. Such sheets may be cast in a cell which has sides capable of bulgin under the weight of the cast thermoplastic material when in the vertical position. In general, a convenient cell may be made from two parallel glass plates which are properly spaced and clamped at the edges. The walls of the cell may be made of polished metal, plastic or any similar material with the proper strength factors and which will not mar or adhere to the surface of the finished sheet. More thermoplastic material is added to the cell than its normal capacity when the walls are parallel. Under the weight of the cast material, the walls of the cell bulge and the greatest bulge 0r deviation occurs at the center of the cell.

Although we prefer to employ the polymeric derivatives of acrylic acid and methacrylic acid, our process is not limited to these materials; All thermoplastic materials, which are capable of being cast into rigid sheets which adhere to the walls of the cell during polymerization and which, when hard, may be conveniently separated from the cell, may be employed. Suitable materials include the following, which may be used alone or in various combinations derivatives of acrylic and methacrylic acids, styrene, acrylamide, chlorinated and otherwise substituted acrylic and methacrylic derivatives, vinyl chloride, vinyl acetate, etc. 3

The cell and its contents are heated until polymerization has been completed. Thereafter, they are cooled and the finished sheet is removed from the cell.

Depending upon the ultimate use to which it is to be put, the sheet may be tinted, or colored, or opacified. Soluble dyes may be used for tinting and coloring and pigments may be used as opacifiers.

Softening agents, coloring matter, catalysts, polymerization regulators or Other modifying substances may be employed when desired.

The drawing illustrates the construction of a cell as used in practicing our invention, and the following is a deecflptlon of our method of formlnzlosxled sheets.

Mg. l is c. View of en empty circular cell with one side-Well removed.

. 3 is e mes-section of a complete cell.

Fig. 8 shows e. cell contelulng a sheet-formlug materiel A to a level ouch as is normally reached in the production of sheets oi uniform. chime.

P15. 4 lo a cross-section clung line o e of Fig. 3 and shows the bulge which occurs in the cell due to the'weight of the contents when the cell is in the vertical poeltlou.

Flea 5 lo :2. cross-section of the some cell, as shown he Flo. 3, which has been eeeleol end plececl in the horizontal ,oooltlon.

Flg. i5 is ecmee-scctlon of at cell ln-vertlcel 13051- tlcu which has been entirely em seuleel.

Flo. i ehosvothe cmupletely llllccl cud sealed cell'ln the horleoutel position, to. poeitlon it remains bulgecl.

Referring to the flames, in which like reference characters deal etc the same ports, two glee-5; sheets 2 erehelcl in position by spacers The edges of the cell ore sealed with binding material 6. 5 represents an ouenieg' threug. whlch p ly-=- m rlzeele mctemle ll; pour-eel to level. 6, teller which ooemmg :3 is closed end teelell.

Relerrms to the drawing, the process oi costme loeclecl sheets of polymeric meterlfl, for e2;- ample,'mo be described as follows: While the cell is held in a, vertical poeltlon, it is filled. with the llquld monomer in which the necessary mod? lying agents hove been cllseolved. Under these conditions, due to the weight of the liquid, wells of the cell will bulge as shown lo 6. lit is preferred to add to the cell volume of llorzlcl whlch is between about 26% cool about 8M2 greater then the volume ol the empty cell when ts wells are parallel. The eseembly is then heat. ed at the boiling point of the monomer in e cult oble oven or bath for a. period of ebout 2 to about 30 minutes, during which the dissolved ere expelled but polymerization is not "oemlltteo to proceed to e great extent. At this poloe, the cell is sealed and is lowered to the horizontal position, as shown in Fig. 7. While the cell is held in this position, the temperature is so adjusted as to cause complete polymerization to take place.

When polymerization is complete, the cell is ollowed to cool to approximately room temperature whereupon the cell walls axe and the locu ed sheet of polymer ls removed.

Although the drawing shows ollculal cells, it is apparent that the cells may equally well be shaped otherwise. For example, a. cell may be formed. of square wells.

As to the specers, which are employee in the construction of the cell, they can be cosostluctell of thermoplastic materiel in which case they are not removed during the polymcrlzetlon; or they may be oi rigid material and be removed at some intermediate stage of the gsolyl zetioxl.

A more thorough unclerstemlme of this lnven tlon may be had from the following examples.

Ecsomple 1 Seven pounds of a mixture of methyl math ecrylate and ethyl ocryla-te, containing 0.84% benzoyl peroxide as a polymerization catalyst and. 0.5% of a release agent, was introduced into a cell inches in diameter, similar to that shown in Fla. 3. The walls bulgecl slightly as shown in Fig. 4.. The cell cud contents were heated to the wee lee boiling point of the contents and boiling Wee (11:31 tlnuecl until all occluded gases had been released. The cell then cooled to (3., closed, scelecl, and gradually lowered to the horizontal position, in which position the Wells were essentially parallel. Heating was continued at 70 to 109 C. for several hours during which polymerization become complete. After being cooled to room tem pereture, the sheet Wes removed from the cell and was found to be slightly thinner in the center than at the edges.

Examine 2 A loaded sheet was prepared of the mixture and in the same manner as recolreeu Example 1, except that eleven pounds of mono merlc mixture was introduced into the cell. Under the weightof the liquid the Walls of the cell bulged as shown in Fig. 6. The cell and contents were heated es in Example 1 to drive of? all occluded gases and thereafter the cell wee closed, sealed, and lowered to the horizontal position where 11: assumed an appearance as shown in Fig. 7. Heating was continued at about 70 C". to about C. for several hours during which polymerimtion was completed. After being cooled to room temperature the sheet was removed from the cell and was found to be definitely thicker at the center than at the edges.

The variation in thickness of the sheets, pro closed in accordance with Examples 1 and 2 respectively, was measured at equidistant gsoints across the middle of each sheet. The inference 1x1 thickness is here tebulel'ed:

I liixcmeies 3 one".

Each ofthe two sheets resulting from. l and 2 was stretched into 9. heu'lisphellc dome by the process describe-j in copeurliug ep l1 lion, Serial No. 436,- 32, new United States Patent No.

fllecl simultaneously with 13111." eppli Lion. The operation cousisteu of heating e sheet until it heel become ductile and. then securely clamping the sheet to the rim of o. cylindrical chamber, after which the pressure within the chamber was reduced b means of a suction pump whereupon. the sheet was forced, by the clifierem tie] gas pressure on its two sides, to assume curved shape. The pressure differential was eclglueted until. the sheet had assumed the shape of u hemisphere, after which the pressure was so regulated as to hold the sheet in the hemispherical shape while the letter cooled to a temperature at which it resisted 2? her deformation.

It was noted thet'tl'le dome, made from the 13. lb. sheet, was substantially uniform in thickness throughout while that made from the 7 lb. sheet was definitely thimlel' and weaker at the crown than at the sides.

The following lot e. will serve to show the great advantage in u cruelty and strength possessed by the dome m e from the loaded sheet over that dome mzule Mom the normal or seven-pound sheet. The thickness of each dome was measured at regular intervals along line drawn across the dome from the center of curvature. Also the strength of the ilomes vies measured by placing olfiereut Weights on the domes and noting the spaced relationship and sealed alone their perimdeflection or-amount of yield of the dome under the weight. The results 01 these tests were as follows: 0

Thickness in inchev-C'ross-sectk'm of the domes se Pt.l m Pt.l PM sees 7-lb.dome .15 .10 .07 .o1 .10 .14 114mm .16 .1! .155 .155 .16 .16

Sir-math of time (stnchesof deflection under 1,000 2,000 4.000 v 000 0.000 s- 's-- s- 's. s.

new .01 .00 .00 .04 .05 .00 ll-ib.d0m ..01 .02

The advantage of produclns non-uniiorm or "loaded" or thick-centered sheets for utilization tormly stron: domes is thus readily apparent.

We claim: 1. 111a process of manuiacturinz thick-centered sheets of organic, polymerized. thermoplastic ms.-

terial which comprises iormin: a cell of two par--v in the production of uniformly thick and uni-r p 4. The process of claim 1 in which the fluid.

eters and which have such strenzth factors that they are capable of bulging under the weight or the contents of the cell when in the vertical position, introducing into said cell while in the vertical position a fluid. polymerizable material or a density eumcient to cause said sheets to bulge and in an amount which is iromabout 20% to about 80% greater in volume than the volume or the cell when empty. thereby causing the walls of said cell to bulze. heating the polymerizable liquid to drive oi! extraneous eases. completely sealine said cell. placing said sealed cell in a horizontal position. hes-tins the polymeriaable material to Meet complete polymerisation thereof. coolinl said material. and removing the thick-centered sheet oi polymerized material from the cell.

2. The process of claim 1 in which the fluid. polymerizable material is a mixture of acrylic and methacrylic esters.

3. The process of claim 1 in which the fluid, polymerizable material is styrene.

' polymerizahle material-is a mixture of methyl allel. flit. resilient sheets of material held in methacrylate and ethyl acrylate.

WILLARD F. BARTOE. WALTER R. BPECK. 

